Macromolecules 1998, 31, 6469-6475
6469
Cyclic Arylene Sulfides: A Novel Synthesis and Ring-Opening Polymerization Eishun Tsuchida,* Kenji Miyatake, and Kimihisa Yamamoto† Department of Polymer Chemistry, Advanced Research Institute for Science and Engineering, Waseda University, Tokyo 169-8555, Japan
Allan S. Hay* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, H3A 2K6, Canada Received February 27, 1998; Revised Manuscript Received July 17, 1998
ABSTRACT: Oxidative polymerization of aryl disulfides has been carried out under high dilution conditions for the preparation of cyclic arylene sulfides. The readily available diphenyl disulfide and dichloro disulfide with diphenyl sulfide, diphenyl ether, or p-xylene are useful monomers for the synthesis of the corresponding cyclic oligo(thio arylene)s in ∼ 69% yield. The cyclization of dichloro disulfide with p-xylene gives cyclic oligo(thio-2,5-dimethyl-1,4-phenylene) (3c) bearing a disulfide bond with the repeating units ranging from 3 to 10. The free radical ring-opening polymerization of the cyclics leads to the formation of linear poly(thio arylene)s. The obtained poly(thio-2,5-dimethyl-1,4-phenylene) (6c) has a melting temperature of 354 °C, which is 71 °C higher than that of the unsubstituted poly(thio phenylene).
Introduction Much effort has been recently expended in preparing macrocyclic aromatic compounds in view of their significance as reactive intermediates for the synthesis of high-performance linear aromatic polymers by ringopening polymerization.1 The advantageous aspects of the ring-opening polymerization over the conventional polycondensation methods are the elimination of solvents, the absence of byproducts, and the rapid formation of high-molecular-weight polymers in high yields. Another noticeable attractive feature in using cyclics is their much lower melt viscosity compared with the polymers, which facilitates reactive injection molding. The ring-opening polymerization of cyclics has also potential applications for composite materials and structural adhesives. For these and other applications, a variety of macrocyclic aromatic compounds, including cyclic esters,2 ethers,3 amides,4 ether ketones,5 and ether ether ketones,6 have been synthesized extensively in the past decade. Most of the synthetic procedures for these cyclics are based on nucleophilic aromatic substitution reactions in a polar solvent at high temperatures. In contrast to these reactions, aromatic electrophilic reations proceed efficiently under moderate conditions and have been utilized for the preparation of linear polymers; however, there have been few reports of taking advantage of the reaction for the synthesis of macrocyclic compounds. The very recent paper by Gibson et al. reported the synthesis of a cyclic ether ketone containing a sulfone group by the Friedel-Crafts acylation reaction.7 In attempts to develop a novel synthetic method for the preparation of high-molecular-weight poly(thio arylene), macrocyclic aromatic sulfides and disulfides have been prepared by several groups.8 We have recently reported our preliminary results on the selec* To whom correspondence should be addressed. E-mail: E.T.,
[email protected]; A.S.H.,
[email protected]. † Present address: Department of Chemistry, Faculty of Science and Technology, Keio University, Yokohama 223, Japan.
tive synthesis of cyclic oligo(thio-1,4-phenylene) through oxidative polymerization of diphenyl disulfide based on an aromatic electrophilic substitution reaction.9 The reaction proceeds under high dilution conditions at room temperature and atmospheric pressure, resulting in the selective formation of cyclic hexamer in high yield. We describe herein our expanded results on the synthesis of cyclic sulfides by oxidative polymerization of dichloro disulfide with aromatics. Several kinds of aromatic cyclic sulfides bearing methyl, ether, and/or disulfide groups have been efficiently prepared. The cyclics are useful monomers for the thermal ring-opening polymerization to obtain linear poly(thio arylene)s. Thermal properties and molecular weights of the polymers are also discussed. Results and Discussion Synthesis and Characterization of Cyclic Oligo(thio arylene)s. The oxidative polymerization of diphenyl disulfide (DPDS) was carried out in acidic dichloromethane solution in the presence of an equimolar amount of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as an oxidant (Scheme 1). To obtain cyclic oligo(thio phenylene), the reaction was done under high dilution conditions by slow addition of the monomer solution into an acidic oxidant solution over a period of 2 days so that the final DPDS concentration should not exceed 0.025 M. The yellow oxidant solution turned immediately to dark green with the addition of DPDS solution and gradually changed to pale brown. The reaction proceeds efficiently at room temperature and usual workup gives a white powder containing the cyclic product as the chloroform-soluble part in 60% yield (Table 1). The chloroform-insoluble part was confirmed to be a linear octakis(thio phenylene) bearing a disulfide bond by MALDI-TOF-MS (matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy) analysis. Our previous study shows that the disulfide bond is at the end of the chain.10 The chloroform soluble part was characterized to be pure cyclic hexakis(thio-
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6470 Tsuchida et al.
Macromolecules, Vol. 31, No. 19, 1998 Scheme 1
Table 1. Oxidative Polymerization of Disulfides for the Preparation of Cyclic Oligo(thio arylene)s yield (%) cyclic
total
cyclic
Mw
Mn
Mw/Mn
Tm (°C)
1 2a 2b 3c
82 83 89 80
60 51 60 69
a 570 600 520
a 490 570 400
a 1.2 1.1 1.3
209 180, 195 122, 144 250, 271
a A single product of hexamer confirmed by MALDI-TOF-MS analysis.
Figure 2. Positive ion MALDI-TOF-MS spectra of cyclic oligo(thio arylene)s prepared by the oxidative polymerization. The peaks marked with solid circles (b) are attributed to the contaminated linear oligomers. The peaks marked with a cross (×) are artifacts of the matrix (dithranol). Figure 1. Effect of DPDS concentration on the total product yield (O) and cyclic hexamer (1) yield (b).
1,4-phenylene) (1). The MALDI-TOF-MS spectrum shows only a singlet corresponding to the correct molecular weight ion signal (M+ ) 648) for the cyclic hexamer with a good signal-to-noise ratio. In addition, gradient HPLC analysis showed the formation of a single product. The cyclic hexamer composed of the thio-1,4-phenylene unit was also well-characterized by IR and 1H and 13C NMR. These spectroscopic data give no indication of the presence of the end phenyl groups. GPC analysis shows a single sharp peak indicating that the cyclic product has Mw of 420 (a degree of polymerization of 4) relative to polystyrene standards.11 The yield of the cyclic depends on the feed concentration of DPDS (Figure 1). At 0.01 M DPDS concentration, the total yield of the oligomeric product was 73%, in which the yield of the cyclic product was 51%. Increasing the DPDS concentration results in an increase in the cyclic yield with a maximum of 60% at 0.025 M. At higher DPDS concentrations yields up to 100% can be obtained along with a significant decrease in the cyclic yield (
